In a laser-type image forming apparatus, there are one or more photoelectric drums that are used in conjunction with the image formation process. If the image forming apparatus is a monochromatic or black and white (B/W) device, then there is typically only one drum used for black (K) image formation. If the image forming apparatus is color, however, there are typically four drums used, one each for black (K), cyan (C), magenta (M), and yellow (Y). During image formation, the photoelectric drum is exposed to a laser, which forms a latent image on the drum. The latent image on the surface of the drum passes by a toner source, such as a developing unit, which attracts toner to the surface of the drum to form a toner image. The toner image is then transferred to an image receiving medium, such as a paper sheet, and the transferred image is fused to the image receiving medium by a fusing unit.
To maintain proper image formation, the image forming apparatus employs a mechanism for cleaning the surface of the photoelectric drums. FIG. 1 is a diagram of a conventional “cleaner-less” drum cleaning system in an image forming apparatus. In a cleaner-less system, there is no cleaning blade or other element contacting the drum 10 to remove toner from its surface. As shown in FIG. 1, the image forming apparatus includes a drum 10, a developer 12, a transfer roller 14, a brush 16, and a charger 18. The drum 10 rotates in a clockwise direction, and the surface of the drum is charged by the charger 18. The charged surface is exposed to a laser (not shown) to generate a latent image. The latent image passes by the developer 12, which transforms the latent image into a toner image. The toner image is then transferred onto an image receiving medium by the transfer roller 14.
As shown in FIG. 1, the vast majority of the toner on the surface of the drum 10 is negatively charged toner 20, although there is some positively charged toner 22. During image formation, the image area (area exposed by the laser) typically has a −60V charge, and the non-image area typically has a −400V charge. The developer 12 is typically biased at a voltage of −250V. As the latent image passes by the developer 12, toner is transferred to the image areas. Any residual toner on the drum 10 in a non-image area is transferred to the developer 12.
As also shown in FIG. 1, the brush 16 is positioned adjacent to the surface of the drum 10. The brush 16 is given a positive voltage bias such as +600V. With the positive bias, the negative toner 20 is collected by the brush 16. However, the brush 16 is not able to collect the positive toner 22, although there are few positively charged toner particles 22. During periods between image forming operations, the bias voltage of the brush 16 can be reversed, which discharges toner from the brush 16 to the drum 10. The discharge to the drum 10 scatters the toner as a layer on the surface of the drum 10, which is then collected by the developer 12. In color image forming apparatuses, however, the toner filming is particularly severe on the color drums and cannot be effectively removed by the developer 12. As a result, the filming often results in the whitening of printed image areas as the film blocks the surface of the drum 10 from being imaged effectively.
It would be desirable to have a design that effectively cleans toner from the drums.